In future development of mobile communication systems, the mechanisms of Device to Device (D2D) discovery and Device to Device (D2D) communication have been introduced in order to better satisfy demands of their subscribers, and to improve the efficiency of exchanging information between devices.
In a Long Term Evolution (LTE) system, communication is controlled centrally by a network in a Device to Network (D2N) transmission mode in FIG. 1, which illustrates the architectural diagram of the system in the D2N transmission mode. As illustrated in FIG. 1, both uplink and downlink data of a User Equipment (UE) (i.e., a terminal) are controlled by an evolved Node B (eNB) (i.e., a base station) to be transmitted and received, where communication between one UE and another UE is forwarded and controlled by the eNB, so there is no direct communication link between the UE and the other UE.
In the versions prior to the LTE Rel-11, mechanisms for triggering a Buffer Status Reporting (BSR) in the architecture of the D2N system include a regular BSR, a periodic BSR, and a padding BSR.
After the base station allocates an uplink resource for the UE, if the size of the uplink resource is exactly sufficient to transmit uplink data, then the UE will transmit the uplink data over the uplink resource without triggering any BSR; or if the uplink resource is not sufficient to transmit all the uplink data, then the UE will preferentially make a regular BSR or a periodic BSR, and the base station will further schedule subsequent transmission according to the amount of uplink data required for the UE carried in the BSR. The UE composes a Media Access Control (MAC) Protocol Data Unit (PDU) so that there is at most one BSR in an MAC PDU, and if there are a number of BSRs triggered concurrently, then the UE will report only the BSR at the highest priority, where the priority of the regular BSR is equal to the priority of the periodic BSR, which is higher than the priority of the padding BSR.
In the LTE system, D2D communication transmission is allowed to be conducted directly between one device and another device proximate to each other in FIG. 2 illustrating the architectural diagram of the system in the D2D transmission mode. As illustrated in FIG. 2, a communication link between one UE and the other UE is a Device to Device (D2D) link, and a cellular communication link between the UE and the eNB is a Device to Network (D2N) link. In a D2D communication resource allocation scheme, the base station allocates a resource for carrying D2D communication scheduling signaling, and a resource for carrying D2D communication data. Thus the UE needs to request the eNB for a D2D transmission resource over the D2N cellular link in a D2D resource request procedure performed through and separate from a D2N BSR procedure.
Mechanisms for triggering a D2D BSR include a D2D regular BSR, a D2D periodic BSR, and a D2D padding BSR. The mechanisms for triggering the D2D regular BSR and the D2D periodic BSR can be the same as the mechanisms for triggering the D2N regular BSR and the D2N periodic BSR, but a new trigger mechanism needs to be introduced for the mechanism for triggering the D2D padding BSR unlike the mechanism for triggering the D2N padding BSR.